Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (SPE) membrane or proton exchange membrane (PEM), to provide ion transport between the anode and cathode.
Fuel cells in general are electrochemical devices that convert the chemical energy of a fuel (hydrogen, methanol, etc.) and an oxidant (air or pure oxygen) in the presence of a catalyst into electricity, heat, and water. Fuel cells produce clean energy throughout the electrochemical conversion of the fuel. Therefore, they are environmentally friendly because of the zero or very low emissions. Moreover, fuel cells are high power systems, generating anywhere from a few watts to hundreds of kilowatts with efficiencies much higher than conventional internal combustion engines. Fuel cells also produce low noise because they have few moving parts.
In proton exchange membrane fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (MEA) in which a solid polymer membrane has an anode catalyst on one face and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of non-porous, electrically conductive elements or plates (i.e., flow field plates). The plates function as current collectors for the anode and the cathode and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive, and gas impermeable. In typical applications, individual fuel cells are stacked in series in order to provide the required level of electrical power.
Embodiments of the conventional electrochemical cell also include hardware components, e.g., plates, for reactant flow separation, current collection, compression and cooling (or heating). A bipolar plate provides multiple functions: (a) distributes reactant flow at the anode or cathode, (b) collects electrical current from operating anode/cathode surfaces, and (c) prevents mixing or cross-over of the anode and cathode reactants in single cells. An assembly of two or more of these single cells is called a stack. A cooling plate (often integral with the bipolar plate) primarily distributes coolant flow in a stack. The number and sizing of single cells in a fuel cell stack is generally selected based on the system power requirements. For convenient assembly and/or dis-assembly of a fuel cell stack with large voltage or power output, multiple sub-stacks or modules, can be combined to form the stack. The modules represent stacks of single cells in some number less than what ultimately results in the completed stack, as is well understood by those of ordinary skill in the art. When the stack forms a PEM fuel cell, the stack is often referred to as a PEM stack.
In a conventional PEM stack assembly, sealing of hardware components and active cells, for effective separation of anode and cathode reactant-flows and prevention of their leakage and intermixing, is a critical technical issue with direct impact on stack performance and reliability. These factors, in addition to sealing system design and design for manufacturability have a direct impact on the overall PEM fuel cell system cost. Leakage or cross-mixing of reactants and coolant between different cells and multiple elements of a single cell is conventionally prevented by compressive or adhesive seals, which in some instances make use of elastomeric and/or adhesive materials.
With reference to FIG. 1A, microseal material 120 may be transferred to a metal bead 122 through a screen 126. The final shape of the traditional microseal 124 is controlled by bead shape (gravity), stencil design, ink viscosity, and surface energy. Unfortunately, as shown in FIGS. 1B-1C, when there is debris 146 from the substrate or the coated electrode on the metal bead 122, the metal bead 122 may buckle in region 150 where the debris 146 is located and/or the microseal material 124 may be damaged. As is known, the UEA 134 having a carbon substrate and a coated electrode is generally assembled between two bipolar plates 123. The bipolar plates 123 are generally joined at the metal bead seal region. However, despite efforts to maintain a clean environment in the assembly process, the carbon substrate and/or the coated electrode from the UEA 134 may be brittle in nature thereby generating some debris 146 which falls on the microseal 124 and metal bead 122 (FIG. 1B) in the assembly process. Such debris 146 can present issues where the metal beads 122 of the two bipolar plates 123 (FIG. 1C) are joined, seal contact is not guaranteed, especially in the regions 150 where the metal bead 122 buckles due to such debris 146. Leaks in the seals can generate overboard leaking wherein gases from the fuel cell stack are leaked into the environment. Given a set of metal properties and metal bead geometry including metal thickness specifications, it may not be possible to improve the buckling load of the metal bead sufficiently, by altering the metal form alone.
Accordingly, there is a need for a method to manufacture a fuel cell which removes debris at the metal bead seal region of the bipolar plates when the bipolar plates for a fuel cell are joined together in order to improve seal contact and increase robustness.